1. Field of the Invention
The present invention generally relates to an optical disc player and, more particularly, to a galvanometer mirror drive for driving a galvanometer mirror for causing a light spot to track a recording track on an optical disc and a detecting system for detecting the amount of movement of the galvanometer mirror.
2. Description of the Prior Art
To accomplish a high speed access to an optical disc, it is getting a very important problem to reduce the weight of an optical head movable unit that is driven during the access. A system wherein a pivotable mirror (galvanometer mirror) supported on a fixed portion is used to accomplish a tracking control is considered an effective solution to this problem. For example, U.S. Pat. No. 4,330,880, issued May 18, 1982, discloses the use of a galvanometer mirror to accomplish a tracking thereby to achieve a high speed seeking. A pivotal mirror drive device which is an important element in accomplishing this tracking control is disposed in an optical disc player on a path extending from a semiconductor laser to an optical disc medium where a light spot is formed so that, when the mirror is pivoted, the light spot can be moved in a direction perpendicular to a recording track on the optical disc.
This known pivotable mirror drive device comprises a leaf spring member, a galvanometer mirror pivotally supported by a housing through the leaf spring member, a drive coil secured to the leaf spring member and a permanent magnet secured to the housing so as to confront the drive coil. When an electric current flows across the drive coil, the drive coil receive an electromagnetic force from the permanent magnet to permit a movable unit, comprised of the drive coil, the leaf spring member and the pivotable mirror, to move. With this system it is possible to move reflected rays of light of the laser beam radiated towards the pivotable mirror and, therefore, the tracking control in the optical disc player can be realized. The tracking control of the light spot by the galvanometer mirror is accomplished by a control of movement of an optical axis along which the rays of light reflected from the pivotable mirror travel, and a mechanical vibratory system in which the pivotable mirror is supported by the leaf spring member is a major element decisive of a tracking servo performance.
U.S. Pat. No. 4,423,496, issued Dec. 27, 1983, discloses the use of a mirror position detecting device which comprises a reflecting plate secured to a back face of the galvanometer mirror and a photo reflector including a light emitting diode and two photodiodes and fixed at a position confronting the reflecting plate such that a pattern of intensity distribution of rays of light reflected from the reflecting plate when the rays of light from the light emitting diode impinge upon the reflecting plate can be moved above the two photodiodes thereby to detect the position of the mirror. According to this reference, an output signal from the mirror position detecting device is utilized as a correction signal with which a position control loop is operated to accomplish a highly stable tracking control servo.
Since the accuracy of tracking control depends on the accuracy of detection of the position of the pivotable mirror, various devices utilizing light have been suggested for detecting the amount of movement of the pivotable mirror. For example, U.S. Pat. No. 4,466,088, issued Aug. 14, 1984, discloses the use of a second reflecting mirror on one side adjacent a reflecting face of the galvanometer mirror to cause monitoring rays of light to be reflected sequentially by the second reflecting mirror, then by the reflecting face of the galvanometer mirror and finally by the second reflecting mirror before a change in amount of light so reflected can be detected. For this mirror position detecting device, various devices have been suggested.
Any one of U.S. Pat. No. 4,556,964, issued Dec. 3, 1985, and U.S. Pat. No. 4,564,757, issued Jan. 14, 1986, discloses an attempt to increase the detection sensitivity of an optical system for a mirror position detection by suppressing interference between the light beam, used for information recording or reproducing, and the light beam used for the position detection.
However, with any of the prior art structures discussed above, there is a problem in that the number of optical component part is so large as to result in an increase of the cost of manufacture. The detection sensitivity required form the detection of amount of movement of the mirror may be expressed in tern of a change in angle of the mirror. For example, to achieve the tracking control in the optical disc player, it is generally required that the amount of movement of the light spot allowed against the track of about 1.6 .mu.m in width is not greater than 0.1 .mu.m and the amount of movement of the pivotable mirror required in such case is about 0.1.degree. at maximum with the signal-to-noise ratio not lower than 40 dB. In other words, the detection sensitivity of 0.001 degree or greater is required, and it has long been considered difficult to achieve such a high sensitivity angle detection.